The present invention relates to a system for the conversion of solar energy. More particularly, the present invention is concerned with a prismatic solar reflector panel and a method of solar tracking for use in a solar energy conversion system.
One of the methods currently being investigated for the generation of electricity from solar energy is the Heliostat-Central Receiver concept, also called the Solar Power Tower System. In this system, flat or slightly concave mirror panels are mounted on rotating pedestals so that each mirror panel tracks the sun to reflect the solar rays to a receiver located at the top of a tower. An array of many such panels are positioned around the tower to focus a large amount of solar energy on the receiver. The receiver functions as a boiler to produce steam or a heated working fluid for use in a conventional turbine-generator power plant. Each set of one or more mirror panels, together with appropriate support structure and a tracking system, is known as a heliostat. Many thousand such heliostats are necessary for the operation of a multi-megawatt power installation, with about one-half of the total cost of such a power system being required for the heliostats.
Current heliostat panels use metallized glass or plastic, in sheet or film, as the reflector material. Silver or aluminum are the most common metals for this application, both requiring protection from oxidation by an overcoating of plastic or glass. Such reflector panels encounter losses due to absorption and scattering of the incident light by the metal reflectors and by the glass or plastic transparent covering. Even highly polished silver absorbs and scatters about six percent of the incident sunlight, and a typical glass covering absorbs and scatters another eight percent of the sunlight as the light passes through the glass before and after being reflected from the metal surface. Thus, the specular reflectance of a second-surface, silvered-glass mirror is typically only about 80 to 86 percent. Silvered plastics generally have similar specular reflectance values. Polished, clear-anodized aluminum sheeting has a specular reflectance much less than for silvered glass, typically about 58 to 75 percent.
In addition to the disadvantage of relatively low specular reflectances, both glass and plastic metallized mirrors are expensive for solar energy reflector panels, typically costing about one dollar per square foot or more in large quantities. Furthermore, plastic mirrors are not durable when exposed to normal outdoor variations in temperature and moisture. The difference in the coefficients of thermal and moisture expansion of the metal and plastic materials used in the panels results in a gradual degeneration of the bond between them. Although glass mirrors are more lasting, the heavy weight and fragile nature of glass are severe disadvantages in most solar energy applications.
A number of different heliostat designs are currently under investigation. Most approaches used a metal frame construction with glass mirrors directly exposed to the wind and weather. In one configuration, four mirror modules are mounted inside a gimbal frame which rotates upon two vertical "I" beams arising from concrete slabs. This approach requires separate alignment for each of the mirror modules in the heliostat. Other applications utilize rectangular or trapezoidal segments in an array mounted on a vertical yoke or pedestal. The individual mirror segments can be pre-focused to some degree, and the entire array is rotatable as a unit about the yoke or pedestal for automatic solar tracking. In another application a plastic air-supported dome is provided to protect a heliostat panel of stretched aluminized polyester film membrane attached to a circular frame. Although this design provides the protection from the elements necessary for an extremely lightweight system, the heliostat performance is reduced roughly 20 percent due to transmission losses in the plastic dome.
In the past, prisms or prismatic plates employing multiple-element arrays have been used primarily for refracting light rays into a spectrum in which the radiation components are separated according to wave length. Prisms have also been used in binoculars and other imaging instruments to provide total internal reflection (TIR) without refraction. This condition occurs only if the angle of incidence of a light ray falls within a limited range when impinging the internal surface of a material having a given index of refraction. A similar total reflection principle has also been applied in other areas not involving prisms. For example, in fiber optics light is transmitted along a hollow glass tube by total reflection. More recently, in the field of solar energy collection, a compound parabolic concentrator has been investigated which uses a trough-like surface for total internal reflection to funnel light within the concentrator to an absorber unit. This type of design appears to be impractical for most solar collection systems because of the prohibitively large dimensions necessary for the concentrator to provide solar energy conversion in useful quantities.